This is the way I build my Janna I am usually successfully with this build. It may not be ideal for you high ELO people. The highest I've ever been was 1456 before the ELO reset so I've never been that great. However I feel like Janna is a viable with simply two main items: RoA and Morello's.

For runes, I go more of an offensive rune build. Getting Magic Pen, Mana Regen, and Ability power. Though Janna isn't exactly made to be an AP carry, giving her some extra offensive ability as a support won't hurt. This rune combo gives Janna a great well rounded support and damage aspect.

Her masteries are basic: 9/0/21. Getting magic penetration and cool downs. I get the reduced death time as Janna is a great support/defender, the more she can be out in the battles the better she is. Not forgetting the flash and clairvoyance buffs!

I give Janna offensive runes and not exactly a support item build; however, Janna is a support. Clairvoyance is a great summoner spell for supports.
Janna only needs flash to escape. Janna is one of the fastest champions in League and should be able to run away with her knock up, slow, shield, push away and flash(if anything).
Flash can also be used offensively by flashing into a team fight and ulting to push the enemies into your team mates

I master out E the shield first. The shield is great for both champions and towers. Defending a tower or being offensive with a carry will both work with the shield. Before moving into a battle always shield the AD carry first if there is no AD carry shield the champion (preferably a carry) that is taking the most damage.
Then I master Q the knock up. It is your number one offensive ability and secondary support ability. Using your knock up to hinder your enemy's escape will help you and your team. Timing is essential on Q. Remember you can always release Q earlier than expected. Try using your Q in awkward places but facing the correct orientation. It'll throw your enemies off and they will not be able to predict and dodge the knock up as easily.
The last skill I master is W, the slow. E will also give you a passive movement speed bonus and she is able to go through units. This ability should help you run away easily. I get one at level 3 to increase my movement speed then I put it off until I master my E. It does have a decent damage output so I recommend you use it when you are harassing a champion in a fight. Once you use it your passive ability will disappear so be aware.
R is always first choice. It is a great support skill. You can use it when you escape from enemies, throwing enemies back from your carries, splitting up the enemy team, healing your allies, and interrupting other champions skills.